Immunoassays are well known for detecting substances which may be involved in immunoreactions. Detection of such a substance, for example in a test solution, may be carried out by incubating the solution with a marker agent and then establishing whether any marker agent is bound to the substance present in the solution. The marker agent may comprise any material which can bind to the substance and which is also labelled with an assay tracer. An assay tracer may comprise a radioactive isotope or some other chemical/biological labelling component upon which the assay may be based.
Immunoassays are available in different formats, for example as an ELISA (i.e. an enzyme-linked immunosorbent assay), such as a single use device (SUD), as a latex agglutination format and as a radioimmunoassay. The ELISA relates the presence of a substance (for example an antigen or antibody) to enzymatic activity. In this type of assay, the substance is detected using a marker agent provided with an enzyme label (see for example U.S. Pat. Nos. 3,791,932; 4,757,134 and 4,833,071). An ELISA also uses an immunoadsorbent composition for capturing the substance of interest prior to the detection thereof.
The SUD typically comprises a membrane carrying a capture antibody to which the antigen under investigation binds. A "sandwich" is then formed using a second marker antibody carrying a suitable label such as an enzyme which is subsequently contacted with a substrate to form a color.
In the latex agglutination format, the antigenic material is adsorbed onto polystyrene beads in the form of latex having a milky appearance. Subsequent addition of the antibody causes agglomeration of the coated latex particles with a consequential change in visual appearance of the medium.
A radioimmunoassay is the same as an ELISA except that the enzyme is replaced by a radioisotope for example iodine 125. Detection is effected using a gamma counter.
Group B streptococcus (GBS) infection constitutes a serious health threat to humans and animals. GBS infection, for example, is the cause of mastitis in dairy herds. Of particular concern, however, is the occurrence of GBS infections at the time of childbirth. Expectant mothers who are carriers of this bacterium are exposed to a risk of postpartum infection, but they may also transfer the infection to their child as the child passes through the birth canal. Prevention of such infection in newborns is possible with preterm testing to identify cervical or vaginal carriage of GBS followed by antibody treatment when necessary (1). Rapid immunodiagnostic testing is preferable to overnight culture in this situation because of the need for the antibiotic treatment before parturition.
Group B streptococcus can be differentiated from other streptococcus by the presence of a group specific polysaccharide antigen ("C" substance) common to the five individual GBS serotypes (2, 3, 4). This common GBS polysaccharide antigen contains L-rhamnose, D-galactose, 2-acetamido-2-deoxy-D-glucose and D-glucitol (5, 6). The entire structure of the GBS polysaccharide antigen has previously been shown to comprise four different oligosaccharide units having the structures I, II, III and IV shown below (5, 6): ##STR1##
The oligosaccharide units I, II, III and lV have been shown to be linked together by a type of phosphodiester linkage (i.e. between 06 of D-glucitol and 06 of D-galactopyranose) to form a complex highly branched multiantennary structure. The overall structure of the GBS polysaccharide antigen can be represented as follows (6). ##STR2##
Immunodiagnostic assays for GBS target the GBS polysaccharide antigen (7). From structural studies, it is known that the GBS polysaccharide antigen has monorhamnose epitopes and 4 trirhamnose epitopes, and it is also known that the terminal monorhamnose group which includes the group of formula .alpha.-L-Rhap-1-, wherein Rhap is rhamnose, is an immunodominant epitope of the GBS polysaccharide antigen (8). However, this monorhamnose group is responsible for much of the cross-reactivity observed with the polysaccharides and antisera of streptococcus B, streptococcus G and streptococcus pneumoniae XXIII (9).
A number of studies have noted that certain commercial tests for GBS polysaccharide antigen, of the ELISA and latex agglutination types, lack sensitivity (10, 11), and may exhibit cross-reactivity with non-GBS bacterial antigens (12). In the case of expectant mothers, false results obtained as a result of such cross-reactivity can lead to unnecessary exposure of the unborn child to antibiotic therapy. Lack of sensitivity to GBS can lead to the more serious situation of failure to administer treatement, with the consequent increased risk of neonatal GBS disease.
The optimum time for screening expectant mothers has been reported to be at the onset of labor. There exists, therefore, a need for a relatively rapid diagnostic test of increased sensitivity and specificity (absence of cross-reativity) to diagnose the presence of GBS, especially in pregnant women immediately prior to childbirth. The present invention seeks to provide a rapid diagnostic test with superior sensitivity and specificity towards GBS.